problem with ClientScript.RegisterStartupScript

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C#

ASP.NET

Javascript

Ajax

well I have 2 pages page1 and page2
from page1 after clicking on grid button loads page2 as window

In page 2 I have 3 control: TextBox and 2 Button Save and Close

here is code below

<script type="text/javascript">
             function returnToParent(result) {
                 var arg = new Object();
                 arg.result = result;
                 arg.text = document.getElementById("txtComment").value;
                 var wnd = GetRadWindow();
                 wnd.close(arg);
             }
             function GetRadWindow() {
                 var mwnd = null;
                 if (window.radWindow) mwnd = window.radWindow;
                 else if (window.frameElement.radWindow) mwnd = 
                          window.frameElement.radWindow;
                 return mwnd;
             }
</script>

<asp:TextBox ID="txtComment" TextMode="MultiLine" runat="server"
     Width="700px" Height="60px" />
<asp:LinkButton ID="lnkSave" runat="server" Text="Save" Width="100%" 
     CssClass="white" onclick="lnkSave_Click" />
<asp:LinkButton ID="lnkClose" runat="server" Text="Close" Width="100%"
     CssClass="white" CausesValidation="false"
     OnClientClick="returnToParent(0); return false;" />

and Code Behind

protected void lnkSave_Click(object sender, EventArgs e) {

            if(!string.IsNullOrEmpty(txtComment.Text))
            {
                // code
                ClientScript.RegisterStartupScript(this.GetType(),
        "CloseWindow", "returnToParent(1)", true);
            }
            else {
               ClientScript.RegisterStartupScript(this.GetType(), 
        "CloseWindow", "returnToParent(1)", true);
            }
}

as you see close button I call javascript function returnToParent(result) and set resalt 0
it warks and does not postback
now I want to do if TextBox.Text is empty that I check it in code behind
if(!string.IsNullOrEmpty(txtComment.Text))

if it's empty it goes to else statement there I try to do it so that not postback with registering script
but there is trouble when I click save button when textbox is empty it postbacks again

How can I do it that not postback?? :(